1. Field of the Invention
The present invention relates to a measured data synchronization system comprising a plurality of measuring instruments (for example, measuring devices and sensors) for measuring objects under measurement and outputting measured data, and a data processing apparatus connected to the plurality of measuring instruments via a signal line and which acquires and processes the measured data output by the measuring instruments. More specifically, the present invention relates to a measured data synchronization system which performs data processing using measured data for which synchronization among measuring instruments is ensured.
2. Description of the Prior Art
If a plurality of measuring instruments are used for measurement and measured data is acquired from these measuring instruments for data processing, it is necessary to ensure the synchronization of measurement results among the measuring instruments. A measured data synchronization system is intended to ensure the synchronization of measured data before data processing. Traditionally, there have been various kinds of systems for ensuring synchronization (for example, refer to the Japanese Laid-open Patent Application 1999-355256).
FIG. 1 illustrates an example of prior art measured data synchronization systems. In FIG. 1, data processing apparatus 10 is a computer or the like and is connected to general-purpose signal line 100. Measuring devices 2A to 2C are measuring instruments, have clocks 21 and synchronization circuits 22, and are connected to general-purpose signal line 100. Clock 21 outputs times. Synchronization circuit 22 is connected to clock 21, as well as to the synchronization circuits 22 of mutually adjacent measuring devices 2A to 2C via dedicated signal lines 200. General-purpose signal line 100 is, for example, an Ethernet (registered trademark). Dedicated signal line 200 is less susceptible to signal deterioration and reliably transmits signals.
Behaviors of such a system as mentioned above are explained here.
First, the behavior of achieving synchronization among the clocks 21 of measuring devices 2A to 2C is explained. Of the clocks 21 of measuring devices 2A to 2C, the clock 21 of the measuring device 2A, for example, is defined as the reference clock. The synchronization circuit 22 of measuring device 2A acquires times from clock 21 and outputs the acquired time as a synchronization signal to the synchronization circuit 22 of measuring device 2B via the dedicated signal line 200. When given an input of the synchronization signal from measuring device 2A, the synchronization circuit 22 of measuring device 2B immediately outputs this synchronization signal to the synchronization circuit 22 of measuring device 2C via dedicated signal line 200. Then, according to the time contained in the synchronization signal, the synchronization circuits 22 of measuring devices 2B and 2C synchronize their respective clocks 21 with the clock 21 of measuring device 2A. Synchronization circuits 22 perform these behaviors as frequently as possible to achieve synchronization.
Next, behaviors wherein measuring devices 2A to 2C perform measurements and data processing apparatus 10 processes data are explained. Data processing apparatus 10 outputs a start measurement command towards measuring devices 2A to 2C via general-purpose signal line 100, thereby enabling measuring devices 2A to 2C to perform measurements. When measurement is completed, measuring devices 2A to 2C append the time of synchronized clocks 21 to the measured data and output the measured data containing the time towards data processing apparatus 10 via general-purpose signal line 100. Moreover, the data processing apparatus uses the measured data and the time contained therein to achieve synchronization among measuring device 2A to 2C before performing data processing.
Another example of prior art systems is explained by referring to FIG. 2.
Note that elements identical to those of FIG. 1 are referenced alike and excluded from the description. In FIG. 2, data processing apparatus 11 is provided in place of data processing apparatus 10. Data processing apparatus 11 has clock 12 and is connected to general-purpose signal line 100. In addition, measuring devices 3A to 3C are provided in place of measuring devices 2A to 2C. Measuring devices 3A to 3C are connected to general-purpose signal line 100.
Behaviors of such a system as mentioned above are explained below. Data processing apparatus 11 outputs a start measurement command to measuring devices 3A to 3C via general-purpose signal line 100, and also retains the time of clock 12 when the command is output. Then, measuring devices 3A to 3C perform measurements according to the start measurement command from data processing apparatus 11. When measurement is completed, the measuring devices output measured data to data processing apparatus 11 via general-purpose signal line 100. Data processing apparatus 11 processes the measured data sent from measuring devices 3A to 3C, assuming that the measured data was acquired at the point of time that the apparatus retains. This type of system configuration is generally referred to as SCADA (Supervisory Control and Data Acquisition).
As described above, the system shown in FIG. 1 ensures synchronization in such a way that synchronization circuits 22 adjust the times of clocks 21 to each other via dedicated signal lines 200. However, each of measuring devices 2A to 2C requires a clock 21 and synchronization circuit 22. Moreover, dedicated signal lines are required to connect synchronization circuits 22. Synchronization circuits 22 require complicated processing (for example, compensation for the time delays of synchronization signals resulting from the lengths of dedicated signal lines 200 or reconstruction of deteriorated waveforms) to achieve synchronization. In addition, dedicated signal line 200 tends to be more expensive than general-purpose signal line 100 because the dedicated signal line is specifically designed to reliably send synchronization signals. In addition, if a dedicated signal Line is extended due to an increase in the number of measuring devices including 2A to 2C or for reasons of the locations where measuring devices 2A to 2C are installed, the waveforms of synchronization signals may deteriorate or the delay time may be prolonged, thereby significantly increasing synchronization errors among measuring devices 2A to 2C. These factors restrict the number of measuring devices, including 2A to 2C, that can be connected or the locations where these measuring devices can be installed.
On the other hand, systems such as the SCADA system shown in FIG. 2 do not achieve synchronization among measuring devices 3A to 3C but use, as a reference, the time when data processing apparatus 11 outputs a command. However, the time required for measuring devices 3A to 3C to acquire measured data or the communication delay involved in data transmission generally differs among measuring devices 3A to 3C: these effects would result in synchronization errors. Needless to say, the synchronization errors become larger as the number of measuring devices including 3A to 3C increases, or depending on the locations where measuring devices 3A to 3C are installed. It becomes especially difficult to synchronize measured data if the sampling frequency is made higher, causing the measurement interval to become shorter.
An object of the present invention is to realize a measured data synchronization system wherein data processing is performed using measured data for which synchronization among measuring instruments is ensured.